The primary achievement of the instant invention resides in the discovery that certain specific salicylimines either in free form or complexed with zinc in the plus 2 valence state will dramatically increase the rate of cure of certain elastomers while simultaneously providing adequate processing safety. In the manufacture of vulcanized rubber articles, significant cost savings can be realized through a reduction of time the article is required to spend in the mold. Through enhanced rates of vulcanization, the in mold time required to meet minimum cure levels (physical properties) can be reduced; however, processing safety (or scorch) of the stock must be maintained.
Vulcanization, or curing, is a term applied to the process where elastomers, natural and synthetic, are treated with certain chemicals to improve their strength and durability. In general, vulcanization will effect the following changes in raw elastomers: increase tensile strength, eliminate tackiness, decrease solubility in various solvents, increase elasticity and decrease temperature sensitivity. These improved properties can be obtained by treating the raw elastomer with sulfur in the presence of other chemicals such as accelerators. It is known that the presence of accelerators in the vulcanization process is desirable since accelerators enable the vulcanization reaction to be carried out in a shorter period of time and at a lower temperature.
If large amounts of accelerators are used in the vulcanization process, the resulting composition contains appreciable amounts of the accelerator or decomposition products thereof. Although the presence of appreciable amounts of accelerators in the final composition is not detrimental, there are applications where the presence of the accelerator is undesirable, for example, accelerators such as the thiazoles, are skin irritants. Thus, consideration of the amount and type of accelerator present in the vulcanized composition is important where the vulcanized composition is to be used in the preparation of fabrics and articles which may come in contact with the skin.
Vulcanization accelerators presently used in the industry also present other problems. For example, dithioacid salts such as dithiocarbamates or thiurams, so called "ultra accelerators", rapidly accelerate vulcanization at conventional vulcanization temperatures but have the problem of prematurely vulcanizing the rubber compositions during processing steps and as mixing or molding due to the heat generated by the mixing which results in the undesired scorching of the rubber. Sulfenamides, so called "delayed action accelerators", do not possess scorching problems, but they tend to retard the rate of vulcanization.
Accordingly, it has become important in the rubber industry to find novel accelerator systems that prevent scorching of the rubber at processing temperatures, but that allow vulcanization to take place rapidly at vulcanization temperatures and to provide final compositions which do not have accelerators bloom, or come to the surface, of the rubber article.
U.S. Pat. No. 2,615,860 is concerned with the protection of synthetic resin compositions from degradation caused by light, especially ultraviolet radiation. This patent discloses the use of metal chelates as stabilizers. Specifically, this patent is concerned with a solid polymer of ethylene and not more than 10% by weight of a metal chelate compound, wherein the metal chelate compound is a nickel derivative of the Schiff's base derived fromm ethylenediamine and salicylaldehyde.
Chemical Abstract 77 (22): 141163 g discloses that several Russian investigators have determined that compounds such as N,N'-disalicylideneethylenediamine chelates of certain metals are vulcanizing agents for chloro butyl rubbers. The abstract discloses that the degree of crosslinking was evaluated by swelling, and that vulcanization by the chelates without zinc oxide imparted enhanced tensile strength to the rubber. This reference does not suggest or disclose the instant invention which is concerned with the use of certain specific chelates of zinc in certain sulfur vulcanizable rubbers as cure rate enhancers.
U.S. Pat. No. 4,139,523 discloses a polyphosphazene composition stabilized against thermal aging comprising (1) a polyphosphazene represented by the general formula: ##STR2## and wherein n represents an integer from 20 up to about 50,000 and X and X.sub.1 each represent a monovalent radical selected from the group consisting of alkoxy, aryloxy and amino; and (2) a stabilizer consisting of an organic compound compatible with said polyphosphazene and present in an amount sufficient to stabilize said polyphosphazene against thermal degradation, said organic compound represented by the structure: ##STR3## wherein W is --CRR'--CRR'--, or: ##STR4## and wherein R and R' are selected from the group consisting of hydrogen, alkyl, cycloalkyl, aryl or mixtures thereof, and M is zinc or magnesium.